如何让Delphi复合控件在子控件获焦时绘制焦点标识边框？

核心思路 #

通过跟踪复合控件内部的焦点状态（自身或子控件获焦），利用Windows主题API绘制与TEdit一致的焦点边框，无需修改子控件的核心实现，工作量较小。

具体实现步骤 #

1. 自定义复合控件基类
   继承自TCustomControl（支持自行绘制的控件基类），添加焦点状态跟踪和主题句柄字段：

   type
     TMyCompositeControl = class(TCustomControl)
     private
       FHasFocus: Boolean;
       FTheme: HTHEME;
       procedure ChildEnter(Sender: TObject);
       procedure ChildExit(Sender: TObject);
       procedure WMSetFocus(var Message: TWMSetFocus); message WM_SETFOCUS;
       procedure WMKillFocus(var Message: TWMKillFocus); message WM_KILLFOCUS;
       procedure WMPaint(var Message: TWMPaint); message WM_PAINT;
       procedure WMThemeChanged(var Message: TMessage); message WM_THEMECHANGED;
     protected
       procedure CreateParams(var Params: TCreateParams); override;
       procedure Loaded; override;
     public
       constructor Create(AOwner: TComponent); override;
       destructor Destroy; override;
     end;
   

2. 初始化与事件绑定
   在构造函数和Loaded方法中，给所有子控件绑定焦点变更事件，同时初始化主题相关字段：

   constructor TMyCompositeControl.Create(AOwner: TComponent);
   begin
     inherited;
     FHasFocus := False;
     FTheme := 0;
     BorderStyle := bsNone;
     Padding.SetBounds(1, 1, 1, 1); // 预留边框空间，避免子控件被遮挡
   end;
   
   procedure TMyCompositeControl.Loaded;
   var
     I: Integer;
   begin
     inherited;
     // 给所有子控件绑定Enter/Exit事件
     for I := 0 to ControlCount - 1 do
     begin
       Controls[I].OnEnter := ChildEnter;
       Controls[I].OnExit := ChildExit;
     end;
   end;
   
   destructor TMyCompositeControl.Destroy;
   begin
     if FTheme <> 0 then
       CloseThemeData(FTheme);
     inherited;
   end;
   

3. 跟踪焦点状态
   通过子控件的焦点事件和自身的焦点消息，更新焦点状态并重绘：

   procedure TMyCompositeControl.ChildEnter(Sender: TObject);
   begin
     FHasFocus := True;
     Invalidate;
   end;
   
   procedure TMyCompositeControl.ChildExit(Sender: TObject);
   var
     I: Integer;
   begin
     FHasFocus := False;
     // 检查是否还有其他子控件保持焦点
     for I := 0 to ControlCount - 1 do
       if Controls[I].Focused then
       begin
         FHasFocus := True;
         Break;
       end;
     Invalidate;
   end;
   
   procedure TMyCompositeControl.WMSetFocus(var Message: TWMSetFocus);
   begin
     inherited;
     FHasFocus := True;
     Invalidate;
   end;
   
   procedure TMyCompositeControl.WMKillFocus(var Message: TWMKillFocus);
   begin
     inherited;
     // 失焦时检查子控件是否仍有焦点
     FHasFocus := False;
     for var I := 0 to ControlCount - 1 do
       if Controls[I].Focused then
       begin
         FHasFocus := True;
         Break;
       end;
     Invalidate;
   end;
   

4. 绘制焦点边框
   利用Windows主题API绘制与系统TEdit一致的焦点边框，兼容无主题环境：

   procedure TMyCompositeControl.WMPaint(var Message: TWMPaint);
   var
     PaintDC: HDC;
     PS: TPaintStruct;
     ClientR: TRect;
   begin
     PaintDC := BeginPaint(Handle, PS);
     try
       // 先绘制控件原有内容（子控件会自行绘制）
       inherited;
   
       // 有焦点时绘制边框
       if FHasFocus then
       begin
         ClientR := ClientRect;
         if ThemeServices.ThemesEnabled then
         begin
           if FTheme = 0 then
             FTheme := OpenThemeData(Handle, 'EDIT');
           if FTheme <> 0 then
           begin
             // 绘制EDIT控件的焦点状态边框
             DrawThemeBackground(FTheme, PaintDC, EDITS_FOCUSED, 0, ClientR, nil);
           end;
         end
         else
         begin
           // 无主题时绘制标准焦点矩形
           InflateRect(ClientR, -1, -1);
           DrawFocusRect(PaintDC, ClientR);
         end;
       end;
     finally
       EndPaint(Handle, PS);
     end;
   end;
   
   procedure TMyCompositeControl.WMThemeChanged(var Message: TMessage);
   begin
     inherited;
     // 主题变更时重新加载主题句柄
     if FTheme <> 0 then
     begin
       CloseThemeData(FTheme);
       FTheme := 0;
     end;
     Invalidate;
   end;
   
   procedure TMyCompositeControl.CreateParams(var Params: TCreateParams);
   begin
     inherited;
     // 设置控件样式，确保能正确接收焦点和绘制
     Params.Style := Params.Style or WS_TABSTOP;
   end;
   

5. 使用说明

   • 将现有复合控件改为继承自TMyCompositeControl
   • 确保子控件的BorderStyle设为bsNone，避免自身显示焦点边框
   • 若复合控件有自定义背景绘制，需调整WMPaint中的绘制顺序，避免边框被覆盖

关键说明 #

• 利用主题API绘制的边框完全匹配系统TEdit的样式（Windows 10的蓝色细边框、Windows 11的圆角边框等）
• 无需修改子控件代码，仅需通过父控件统一处理焦点状态
• 兼容启用/禁用Runtime Themes的场景

内容的提问来源于stack exchange，提问作者William Tucker